Foamed polystyrenes are employed to an increasing extent in many fields, above all in the building, construction and packaging industries. In many cases it is desired to decrease the flammability of such products by incorporating a flame retardant into them.
It is common to use brominated aliphatics in the foamed polystyrene industry, with hexabromocyclododecane (HBCD) being the most commonly used flame retardant in foamed styrene polymers. The vapor phase mode of action of brominated organic flame retardants relies to a great extent on their thermal stability in relation to that of the polymer. It is desirable to have a flame retardant compound whose thermal stability is close to that of the polymer. This mainly explains the high efficiency of the brominated aliphatics, and among them HBCD, in imparting flame retardant properties to the cellular and foamed polymer materials.
The process for the production of foamed polystyrene, especially extruded polystyrene (XPS), is very sensitive to the quality of the HBCD due to the relatively low thermal stability of HBCD and some of the typical impurities in it. It is extremely important that the flame retardant chosen for foamed polystyrene has good thermal stability. Hydrogen bromide formed as a result of the thermal decomposition of HBCD during the processing/foaming of polystyrene will adversely affect the physical properties of the foamed polymer product. In addition, the HBr formed may cause corrosion of the metal equipment with which the hot blend comes into contact during the process. Furthermore, the industry aims at increasing the operating temperatures, for higher productivity of the process. In order to suppress such undesirable, early, decomposition and to optimize the performance, HBCD usually needs to be stabilized by the addition of a variety of metal-organic and epoxy compounds, in order to allow the processing of HBCD at higher temperatures and for a longer period.
In view of the above, it can be seen that a need exists for bromine-containing compounds which would be efficient fire retardants for foamed polystyrenes while being more thermally stable than HBCD and other known aliphatic bromine-containing compounds both during the production of the foamed polystyrenes and their processing and scrap recycling.
A Dow patent document, WO 91/19758, describes the limited fire retardancy of HBCD, and discloses the use of a mixture of aliphatic bromine compounds, especially HBCD and aromatic bromine compounds such as decabromodiphenyl ether, as flame retardants for polystyrene foams. Another Dow patent, U.S. Pat. No. 6,579,911, discloses an application of HBCD, phosphorous compounds and flow promoters, to improve the flame retardant efficiency of HBCD. The patent emphasizes that, typically, only brominated aliphatic compounds are utilized with styrene-based foams, with HBCD being the most common.
US 2005/0043464 discloses topical application of brominated aromatic compounds, used as additives to beads of polystyrene in a process for making expanded polystyrene molded patterns in lost foam aluminum castings. The brominated compounds accelerate depolymerization of the polystyrene by the liberation of bromine radicals, which reduce the viscosity of the liquid polystyrene.
U.S. Pat. Nos. 5,639,799 and 5,717,001 describe methods of improving the thermal stability of HBCD for application in styrenic polymer foam compositions.
It is, therefore, an object of the present invention to provide novel bromine-containing fire retardants, which have both excellent thermal stability and good fire-retardancy properties, particularly when incorporated in foamed polystyrene.
It is another object of the present invention to provide such novel fire retardants of suitable thermal stability against dehydrobromination both during the production of the foamed polystyrenes and their processing.
It is yet another object of the present invention to provide a flameproof foamed polystyrene formulation, which contains such bromine-containing fire retardants.
It is yet another object of the present invention to provide use of the novel compounds of the invention and mixtures thereof, as flame-retarding agents in polymeric materials, particularly in foamed polystyrenes.
The present invention provides novel polybrominated bisaryl compounds containing bromomethyl or bromomethylene groups which are capable of imparting highly satisfactory flame-retarding qualities to foamed polystyrenes, while being thermally stable against dehydrobromination both during the production of the foamed polystyrenes and their processing. The invention further provides foamed polystyrene compositions containing the said novel polybrominated bisaryl bromomethyl/bromomethylene compounds and mixtures thereof that exhibit excellent fire retardancy.
Other objects and advantages of the invention will become apparent as the description proceeds.